This invention is concerned with reducing occurrences of breakage of television cathode ray tube glass bulbs and bulb components resulting from various temperature changes introduced in manufacturing and salvage processes.
Color television cathode ray tube bulbs include two major components -- (i) a glass front panel for, among other things, supporting arrays of cathodo-luminescent phosphor deposits and (ii) a glass funnel member whose large end is eventually sealed to the face panel and whose small end houses at least one electron beam source. Black and white television cathode ray tube bulbs, although taking on the same basic final exterior appearance as color bulbs, do not comprise separate components, but rather are produced as a single integral unit.
During the various processes of cathode ray tube manufacture, glass bulbs and glass bulb components undergo numerous temperature variations which cause glass breakage with the obvious consequence of decreased yield and thus increased manufacturing cost. For instance, color television front panels prior to application of the phosphor arrays, are heated in order to anneal or stress relieve the glass (commonly known as compaction). Also, following application of phosphor arrays and a lacquer film, front panels are subjected to bake-outs, i.e., the front panel is raised to a high temperature which drives out volatile materials. Black and white tubes undergo a similar bake-out process.
A further heating procedure is required to frit seal the front panel to the funnel. The final heating of color and black and white bulbs usually occurs in the thermal exhaust cycle during which the bulb is heated in an oven while being evacuated. However, the thermal exhaust cycle is not always the final heating procedure of the color television cathode ray tube manufacturing process. If, for some reason, the bulb is reopened, as in thermal shock processes to separate and salvage the bulb or tube components, the glass components are again subjected to temperature cycling.
Following each of these heating procedures the bulb or its components must be cooled. It is during this cool down that the fractures generally occur. During cool down, the glass goes into a tension mode and the glass is more likely to fracture if it has some exposed surface area of especially low tensile strength such as a scratch, a localized weakness or other surface defect.
Prior art attempts to reduce occurrences of breakage have included increasing the length of time over which the glass bulbs or components are cooled. This obviously results in increased manufacturing time and increased cost.